1. Field of the Invention
The present invention relates to x-ray imaging. More particularly, an alignment method is provided for a digital x-ray imager suitable for use in medical diagnostic applications.
2. The Prior Art
In x-ray imaging, it is generally desirable to minimize x-ray exposure to the amount required to achieve acceptable image quality. For medical diagnostic imaging the goal is to keep the x-ray exposure of the patient to the minimum needed. For other applications such as industrial inspection or veterinary studies, x-ray source (tube and generator) life is limited. Here again it is desirable to use no more exposure than needed.
The x-ray imaging set up usually comprises at least an x-ray source, a patient (or other object under study), and a detector. The origin of x-rays within the x-ray source is generally on the order of 1 mm. The distance is small relative to the distance between the source and the detector (on the order of 1 m).
It is desirable to have an x-ray quality that is as good as possible to enhance the utility of the x-ray image. Often antiscatter grids are used to improve x-ray image quality. Typically, the grid and detector are attached to form a detector/grid assembly. The antiscatter grid is generally formed from alternating strips of x-ray opaque material and x-ray transmissive materials. Lead may be used as the x-ray opaque material and plastics, aluminum or fiber may be used as the x-ray transmissive material. Two dimensional arrangements of x-ray opaque and transmissive materials (or air) are possible also. In either arrangement, the grid substantially transmits unscattered (primary) x-rays and substantially absorbs x-rays scattered by the patient or object under study, thereby preventing the scattered x-rays from degrading image quality. This function is accomplished by aligning the strips parallel to the primary x-rays. In some applications, for example, portable x-ray examinations, a patient is examined while prone in a hospital bed. In these applications, the detector/grid is not mechanically aligned perpendicular to the source, which can degrade the grid""s functionality.
Dr. Heber MacMahon, at the University of Chicago, uses a technique involving alignment bars. In his technique, small alignment bars are attached to the front and back sides of the antiscatter grid. The alignment bars may be about 1 mm by 10 mm in area and 0.1 to 1 mm in thickness. The bars are substantially aligned with each other on opposite sides of the same septa of the grid. In other words, they are disposed on each side of the grid and overlap at least one of the spaces formed by the x-ray transmissive grid parts. The detector typically comprises either film or photostimulable phosphor. If the detector is not aligned correctly, the two alignment bars appear in different positions in the image. Misalignment correlates with lower quality images.
However, this alignment information is obtained only after the detector is removed and the latent image is developed. Hence, it cannot be used to improve the image in situ. At best, the alignment information can be used only to indicate after the fact whether the image quality is good. Additional images, resulting in more x-ray exposure, may need to be taken if the image quality is poor.
Hence, a method using alignment information to improve the first diagnostic image is still needed. Also needed is a method that reduces the amount of x-ray exposure required to achieve acceptable image quality.
A digital x-ray imager method is provided for use, for example, in medical diagnostic applications. The method is particularly useful in planar detectors used in portable imaging applications. In such applications, the detector surface is not mechanically constrained to be perpendicular to the central x-ray source beam. The method requires only an insignificant increase in x-ray exposure. The method also achieves a substantially optimal quality diagnostic image in digital x-ray imaging systems. A digital x-ray imaging system is provided including an x-ray source and a detector. An antiscatter grid is attached to the detector and disposed between the detector and the object under study, for example, a patient. The grid has at least one pair of substantially x-ray opaque alignment bars. Preferably, the antiscatter grid has alternating strips of x-ray opaque material (lead) and x-ray transmissive material (for example, plastic, aluminum, fiber or air). The grid has front and back surfaces on which the alignment bars are disposed, one of each pair on each surface.
A low exposure non-diagnostic x-ray image is taken with a dose sufficient to create an image of the alignment bars on the object. For example, the low x-ray dose may be about 0.001 to 0.01 of that used for the diagnostic x-ray image. The relative position of the alignment bars on the image is measured, for example, manually or by a computer algorithm. The relative angle of the detector to the x-ray source is adjusted to align the grid with the x-ray source. The adjustment required is the arc tangent of the distance between the alignment bars in the image divided by the antiscatter grid thickness. A diagnostic x-ray exposure image is then taken of the object. Alternatively, a second low exposure image may be taken to confirm alignment prior to taking the diagnostic x-ray exposure image.